Change that, and the currency shown after the price changes. It should work, so check yopu're changing the right file, and it's uploaded to the right place etc. e.g. change the currency symbol to ! and see if that change shows up.

To set the symbol before the price, you should also have:

Currency::setCurrencySymbol('$');

While we're on the subject, Europeans note that to get a non-ascii currency symbol such as the UK pound sign, try saving the _config.php file in UTF-8 format.